Cationic electrodeposition coating compositions have excellent application workability and form a coating film with excellent corrosion resistance. They have thus been widely used for, for example, automobile parts, electrical equipment parts, and other industrial machinery, which are required to have such properties.
In general, cationic electrodeposition coating compositions are provided in the form of a mixture of two components, i.e., a resin emulsion component in which resin components comprising a cationic resin (e.g., an amino group-containing epoxy resin) and a curing agent (also called “crosslinking agent”; e.g., a blocked polyisocyanate compound) are mixed and dispersed in an aqueous medium, and a pigment dispersion paste component containing a pigment dispersed with a resin for pigment dispersion. Such a coating composition is used for a coating bath, and a current is applied using a substrate as a cathode and the counter electrode as anode to form a deposited coating film on the substrate. The deposited coating film is heated to form a crosslink-cured coating film.
The heating temperature in the coating film formation described above is commonly higher than 160° C. However, to reduce energy costs, there is demand for performing heating at low temperature (80 to 160° C., and preferably 80 to 130° C.). This is called “low-temperature baking.”
The low-temperature baking is commonly performed using a low-temperature curable blocked polyisocyanate compound as a curing agent. For example, PTL 1 (Claim 3) discloses that low-temperature curing is performed using an oxime-blocked isocyanate-containing cationic electrodeposition coating composition. PTL 2 (Claim 7) discloses a low-temperature-baking electrodeposition coating composition that is baked at a temperature of 100 to 160° C. and that oxime-blocked and lactam-blocked polyisocyanate compounds can dissociate (undergo a reaction) at relatively low temperature. PTL 3 discloses that a self-crosslinking resin containing a specific blocked isocyanate group can be cured at a low temperature of 120° C. or less and can also be used as a cationic electrodeposition coating composition.
However, electrodeposition coating compositions whose reactivity at low temperature is enhanced may have unsatisfactory long-term storage stability (bath stability), resulting in, for example, poor finished appearance and corrosion resistance of the coating film.
PTL 4 discloses a method for forming a coating film by low-temperature baking, comprising performing electrodeposition coating using a base resin (an amine-added epoxy resin), performing electrodeposition coating using an aqueous dispersion of a blocked polyisocyanate curing agent to which water dispersibility is imparted, and drying the resulting coating film by heating at 60 to 150° C. Since coating compositions and coating are separately used for the base resin and the curing agent, the storage stability (bath stability) of the coating compositions is improved. However, the curing agent may not be homogeneously present in the coating film, resulting in, for example, poor corrosion resistance. In addition, since this method requires additional coating and washing steps, it is necessary to provide additional equipment.